This section is surprisingly small because the rest of the FAQ is littered with answers involving regular expressions.
For example,
decoding a URL and checking whether something is a number are handled with regular expressions,
but those answers are found elsewhere in this document (in perlfaq9: ``How do I decode or create those %-encodings on the web'' and perlfaq4: ``How do I determine whether a scalar is a number/whole/integer/float'',
to be precise).

The /x modifier causes whitespace to be ignored in a regex pattern (except in a character class), and also allows you to use normal comments there, too. As you can imagine, whitespace and comments help a lot.

While we normally think of patterns as being delimited with / characters, they can be delimited by almost any character. perlre describes this. For example, the s/// above uses braces as delimiters. Selecting another delimiter can avoid quoting the delimiter within the pattern:

Either you don't have more than one line in the string you're looking at (probably), or else you aren't using the correct modifier(s) on your pattern (possibly).

There are many ways to get multiline data into a string. If you want it to happen automatically while reading input, you'll want to set $/ (probably to '' for paragraphs or undef for the whole file) to allow you to read more than one line at a time.

Read perlre to help you decide which of /s and /m (or both) you might want to use: /s allows dot to include newline, and /m allows caret and dollar to match next to a newline, not just at the end of the string. You do need to make sure that you've actually got a multiline string in there.

For example, this program detects duplicate words, even when they span line breaks (but not paragraph ones). For this example, we don't need /s because we aren't using dot in a regular expression that we want to cross line boundaries. Neither do we need /m because we aren't wanting caret or dollar to match at any point inside the record next to newlines. But it's imperative that $/ be set to something other than the default, or else we won't actually ever have a multiline record read in.

Just to show that C programmers can write C in any programming language, if you prefer a more C-like solution, the following script makes the substitution have the same case, letter by letter, as the original. (It also happens to run about 240% slower than the Perlish solution runs.) If the substitution has more characters than the string being substituted, the case of the last character is used for the rest of the substitution.

You can use the POSIX character class syntax /[[:alpha:]]/ documented in perlre.

No matter which locale you are in, the alphabetic characters are the characters in \w without the digits and the underscore. As a regex, that looks like /[^\W\d_]/. Its complement, the non-alphabetics, is then everything in \W along with the digits and the underscore, or /[\W\d_]/.

The Perl parser will expand $variable and @variable references in regular expressions unless the delimiter is a single quote. Remember, too, that the right-hand side of a s/// substitution is considered a double-quoted string (see perlop for more details). Remember also that any regex special characters will be acted on unless you precede the substitution with \Q. Here's an example:

Using a variable in a regular expression match forces a re-evaluation (and perhaps recompilation) each time the regular expression is encountered. The /o modifier locks in the regex the first time it's used. This always happens in a constant regular expression, and in fact, the pattern was compiled into the internal format at the same time your entire program was.

Use of /o is irrelevant unless variable interpolation is used in the pattern, and if so, the regex engine will neither know nor care whether the variables change after the pattern is evaluated the very first time.

/o is often used to gain an extra measure of efficiency by not performing subsequent evaluations when you know it won't matter (because you know the variables won't change), or more rarely, when you don't want the regex to notice if they do.

While this actually can be done, it's much harder than you'd think. For example, this one-liner

perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c

will work in many but not all cases. You see, it's too simple-minded for certain kinds of C programs, in particular, those with what appear to be comments in quoted strings. For that, you'd need something like this, created by Jeffrey Friedl and later modified by Fred Curtis.

Historically, Perl regular expressions were not capable of matching balanced text. As of more recent versions of perl including 5.6.1 experimental features have been added that make it possible to do this. Look at the documentation for the (??{ }) construct in recent perlre manual pages to see an example of matching balanced parentheses. Be sure to take special notice of the warnings present in the manual before making use of this feature.

CPAN contains many modules that can be useful for matching text depending on the context. Damian Conway provides some useful patterns in Regexp::Common. The module Text::Balanced provides a general solution to this problem.

One of the common applications of balanced text matching is working with XML and HTML. There are many modules available that support these needs. Two examples are HTML::Parser and XML::Parser. There are many others.

Most people mean that greedy regexes match as much as they can. Technically speaking, it's actually the quantifiers (?, *, +, {}) that are greedy rather than the whole pattern; Perl prefers local greed and immediate gratification to overall greed. To get non-greedy versions of the same quantifiers, use (??, *?, +?, {}?).

Notice how the second substitution stopped matching as soon as it encountered "y ". The *? quantifier effectively tells the regular expression engine to find a match as quickly as possible and pass control on to whatever is next in line, like you would if you were playing hot potato.

To do this, you have to parse out each word in the input stream. We'll pretend that by word you mean chunk of alphabetics, hyphens, or apostrophes, rather than the non-whitespace chunk idea of a word given in the previous question:

Avoid asking Perl to compile a regular expression every time you want to match it. In this example, perl must recompile the regular expression for every iteration of the foreach() loop since it has no way to know what $pattern will be.

The qr// operator showed up in perl 5.005. It compiles a regular expression, but doesn't apply it. When you use the pre-compiled version of the regex, perl does less work. In this example, I inserted a map() to turn each pattern into its pre-compiled form. The rest of the script is the same, but faster.

For more details on regular expression efficiency, see Mastering Regular Expressions by Jeffrey Freidl. He explains how regular expressions engine work and why some patterns are surprisingly inefficient. Once you understand how perl applies regular expressions, you can tune them for individual situations.

Ensure that you know what \b really does: it's the boundary between a word character, \w, and something that isn't a word character. That thing that isn't a word character might be \W, but it can also be the start or end of the string.

It's not (not!) the boundary between whitespace and non-whitespace, and it's not the stuff between words we use to create sentences.

In regex speak, a word boundary (\b) is a "zero width assertion", meaning that it doesn't represent a character in the string, but a condition at a certain position.

For the regular expression, /\bPerl\b/, there has to be a word boundary before the "P" and after the "l". As long as something other than a word character precedes the "P" and succeeds the "l", the pattern will match. These strings match /\bPerl\b/.

"Perl" # no word char before P or after l
"Perl " # same as previous (space is not a word char)
"'Perl'" # the ' char is not a word char
"Perl's" # no word char before P, non-word char after "l"

These strings do not match /\bPerl\b/.

"Perl_" # _ is a word char!
"Perler" # no word char before P, but one after l

You don't have to use \b to match words though. You can look for non-word characters surrrounded by word characters. These strings match the pattern /\b'\b/.

"don't" # the ' char is surrounded by "n" and "t"
"qep'a'" # the ' char is surrounded by "p" and "a"

These strings do not match /\b'\b/.

"foo'" # there is no word char after non-word '

You can also use the complement of \b, \B, to specify that there should not be a word boundary.

In the pattern /\Bam\B/, there must be a word character before the "a" and after the "m". These patterns match /\Bam\B/:

"llama" # "am" surrounded by word chars
"Samuel" # same

These strings do not match /\Bam\B/

"Sam" # no word boundary before "a", but one after "m"
"I am Sam" # "am" surrounded by non-word chars

Once Perl sees that you need one of these variables anywhere in the program, it provides them on each and every pattern match. That means that on every pattern match the entire string will be copied, part of it to $`, part to $&, and part to $'. Thus the penalty is most severe with long strings and patterns that match often. Avoid $&, $', and $` if you can, but if you can't, once you've used them at all, use them at will because you've already paid the price. Remember that some algorithms really appreciate them. As of the 5.005 release, the $& variable is no longer "expensive" the way the other two are.

You use the \G anchor to start the next match on the same string where the last match left off. The regular expression engine cannot skip over any characters to find the next match with this anchor, so \G is similar to the beginning of string anchor, ^. The \G anchor is typically used with the g flag. It uses the value of pos() as the position to start the next match. As the match operator makes successive matches, it updates pos() with the position of the next character past the last match (or the first character of the next match, depending on how you like to look at it). Each string has its own pos() value.

Suppose you want to match all of consective pairs of digits in a string like "1122a44" and stop matching when you encounter non-digits. You want to match 11 and 22 but the letter <a> shows up between 22 and 44 and you want to stop at a. Simply matching pairs of digits skips over the a and still matches 44.

$_ = "1122a44";
my @pairs = m/(\d\d)/g; # qw( 11 22 44 )

If you use the \G anchor, you force the match after 22 to start with the a. The regular expression cannot match there since it does not find a digit, so the next match fails and the match operator returns the pairs it already found.

$_ = "1122a44";
my @pairs = m/\G(\d\d)/g; # qw( 11 22 )

You can also use the \G anchor in scalar context. You still need the g flag.

$_ = "1122a44";
while( m/\G(\d\d)/g )
{
print "Found $1\n";
}

After the match fails at the letter a, perl resets pos() and the next match on the same string starts at the beginning.

You can disable pos() resets on fail with the c flag. Subsequent matches start where the last successful match ended (the value of pos()) even if a match on the same string as failed in the meantime. In this case, the match after the while() loop starts at the a (where the last match stopped), and since it does not use any anchor it can skip over the a to find "44".

For each line, the PARSER loop first tries to match a series of digits followed by a word boundary. This match has to start at the place the last match left off (or the beginning of the string on the first match). Since m/ \G( \d+\b )/gcx uses the c flag, if the string does not match that regular expression, perl does not reset pos() and the next match starts at the same position to try a different pattern.

While it's true that Perl's regular expressions resemble the DFAs (deterministic finite automata) of the egrep(1) program, they are in fact implemented as NFAs (non-deterministic finite automata) to allow backtracking and backreferencing. And they aren't POSIX-style either, because those guarantee worst-case behavior for all cases. (It seems that some people prefer guarantees of consistency, even when what's guaranteed is slowness.) See the book "Mastering Regular Expressions" (from O'Reilly) by Jeffrey Friedl for all the details you could ever hope to know on these matters (a full citation appears in perlfaq2).

The problem is that grep builds a return list, regardless of the context. This means you're making Perl go to the trouble of building a list that you then just throw away. If the list is large, you waste both time and space. If your intent is to iterate over the list, then use a for loop for this purpose.

In perls older than 5.8.1, map suffers from this problem as well. But since 5.8.1, this has been fixed, and map is context aware - in void context, no lists are constructed.

Starting from Perl 5.6 Perl has had some level of multibyte character support. Perl 5.8 or later is recommended. Supported multibyte character repertoires include Unicode, and legacy encodings through the Encode module. See perluniintro, perlunicode, and Encode.

If you are stuck with older Perls, you can do Unicode with the Unicode::String module, and character conversions using the Unicode::Map8 and Unicode::Map modules. If you are using Japanese encodings, you might try using the jperl 5.005_03.

Finally, the following set of approaches was offered by Jeffrey Friedl, whose article in issue #5 of The Perl Journal talks about this very matter.

Let's suppose you have some weird Martian encoding where pairs of ASCII uppercase letters encode single Martian letters (i.e. the two bytes "CV" make a single Martian letter, as do the two bytes "SG", "VS", "XX", etc.). Other bytes represent single characters, just like ASCII.

Now, say you want to search for the single character /GX/. Perl doesn't know about Martian, so it'll find the two bytes "GX" in the "I am CVSGXX!" string, even though that character isn't there: it just looks like it is because "SG" is next to "XX", but there's no real "GX". This is a big problem.

This succeeds if the "martian" character GX is in the string, and fails otherwise. If you don't like using (?<!), a zero-width negative look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]).

It does have the drawback of putting the wrong thing in $-[0] and $+[0], but this usually can be worked around.

Alternatively, since you have no guarantee that your user entered a valid regular expression, trap the exception this way:

if (eval { $line =~ /$pattern/ }) { }

If all you really want to search for a string, not a pattern, then you should either use the index() function, which is made for string searching, or if you can't be disabused of using a pattern match on a non-pattern, then be sure to use \Q...\E, documented in perlre.

This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.

Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.